The present invention relates generally to the structure and use of radio frequency electrosurgical probes for the treatment of solid tissue. More particularly, the present invention relates to an electrosurgical probe having multiple tissue-penetrating electrodes which are deployed in an array to treat large volumes of tissue, particularly for tumor treatment.
The delivery of radio frequency energy to target regions within solid tissue is known for a variety of purposes of particular interest to the present invention, radio frequency energy may be delivered to diseased regions in target tissue for the purpose of tissue necrosis. For example, the liver is a common depository for metastases of many primary cancers, such as cancers of the stomach, bowel, pancreas, kidney and lung. Electrosurgical probes for deploying multiple electrodes have been designed for the treatment and necrosis of tumors in the liver and other solid tissues. See, for example, the electrosurgical probe described in published PCT application WO 96/29946.
The probes described in WO 96/29946 comprise a number of independent wire electrodes which are extended into tissue from the distal end of a cannula. The wire electrodes may then be energized in a monopolar or bipolar fashion to heat and necrose tissue within a precisely defined volumetric region of target tissue. In order to assure that the target tissue is adequately treated and limit damage to adjacent healthy tissues, it is desirable that the array formed by the wire electrodes within the tissue be precisely and uniformly defined. In particular, it is desirable that the independent wire electrodes be evenly and symmetrically spaced-apart so that heat is generated uniformly within the desired target tissue volume. Such uniform placement of the wire electrodes is difficult to achieve when the target tissue volume has non-uniform characteristics, such as density, tissue type, structure, and other discontinuities which could deflect the path of a needle as it is advanced through the tissue.
Referring now to FIGS. 1-5, a shortcoming of electrosurgical probes of the type described in WO 96/29946 will be discussed. Such electrosurgical probes 10 typically comprise a cannula 12 having a plurality of resilient, pre-shaped electrodes 14 therein. The electrodes 14 may be mounted at the distal end of a reciprocatable shaft 16, and the electrodes 14 will be shaped so that they assume an arcuate shape to produce an everting array when the electrodes are advanced from the cannula 12 into solid tissue, as illustrated in FIGS. 4 and 5. With prior electrode probes, such as the illustrated probe 12, the electrodes 14 have been received within lumen 18 of the cannula 12. The electrodes have had circular cross-sections, and no provisions have been made to maintain the individual electrodes 14 in any particular ordered fashion within the cannula. Usually, a random pattern of electrodes 14 exist within the cannula 12, as shown in FIGS. 1 and 2. When electrodes 14 are initially present in such a random pattern (i.e. prior to distal deployment into tissue), the electrodes will adopt a similar random pattern or configuration when first entering into tissue T. When the electrode pattern is non-uniform at the time of first entering into tissue, the non-uniformity will be propagated as the electrodes are fully deployed, as illustrated in FIG. 4. Such a random, irregular pattern is undesirable since it results in non-uniform heating and tissue necrosis.
It would be desirable to provide improved electrosurgical probes of the type described in WO 96/29946, where the individual electrodes 14xe2x80x2 are maintained in a uniform pattern within the cannula 12, as illustrated in FIG. 3. In particular, the electrodes 14xe2x80x2 should be equally circumferentially spaced-apart and preferably axially aligned with each other within the cannula so that they will follow uniform, equally spaced-apart lines of travel as they penetrate into tissue, as shown in FIG. 5. It will be appreciated that the initial point at which the electrodes penetrate tissue is critical to maintain proper spacing of the electrodes as they penetrate further into the tissue. Should electrodes be misaligned when they first enter the target (i.e. emerge from the cannula) tissue, they will almost certainly remain misaligned as they penetrate further into the tissue. Moreover, the individual electrodes will generally not be steerable or capable of being redirected within the tissue, so there are few options for correcting the configuration after the needles have first penetrated into the tissue. In contrast, by properly aligning the electrodes prior to and at the time they first enter into tissue from the cannula, the proper electrode pattern can be assured as the electrodes deploy radially outwardly into the tissue. It would be still further desirable to provide electrosurgical probes and methods for their deployment which would provide for improved propagation through tissue having non-uniform characteristics. Even when the electrodes are disposed in a symmetrical pattern at the outset of deployment, the electrode paths can be deflected or deviated when the electrodes encounter relatively hard or dense regions within the tissue. It would be beneficial if the electrodes were capable of passing through such regions with minimum or no deflection.
For these reasons, it would be desirable to provide improved electrosurgical probes having multiple, tissue-penetrating electrodes. In particular, it would be desirable to provide improved electrosurgical probes and tissue ablation apparatus of the type described in WO 96/29946, where the electrodes are configured within the probes so that they deploy in a uniform, evenly spaced-apart manner as they penetrate into tissue to be treated, thus overcoming at least some of the shortcomings noted above.
The present invention provides both apparatus and methods for the electrical treatment of a specific region within solid tissue, referred to hereinafter as a xe2x80x9ctreatment region.xe2x80x9d The apparatus and methods rely on introducing a plurality of electrodes, usually being at least three electrodes, to a target site within the treatment region and thereafter deploying the electrodes into a three-dimensional array and preferably in a configuration which conforms to or encompasses the entire volume of the treatment region, or as large a portion of the volume of the treatment region as possible. The present invention particularly provides for uniform deployment of the electrodes within the solid tissue. By xe2x80x9cuniform deployment,xe2x80x9d it is meant that adjacent electrodes are evenly spaced-apart from each other and that pairs of adjacent electrodes are spaced-apart in a repeating, uniform pattern so that the application of electrical current through the electrodes will result in generally uniform heating and necrosis of the entire tissue volume being treated. Usually, the treatment current is radio frequency (RF) current which may be applied to the tissue in a monopolar or bipolar fashion, as described in more detail below.
Apparatus according to the present invention comprises a probe system for penetrating a plurality of electrodes into tissue. The probe system includes a cannula having a proximal end, a distal end, and a lumen extending at least to the distal end, and usually from the proximal end to the distal end. The individual electrodes are resilient and pre-shaped to assume a desired configuration when advanced into tissue. Usually, the individual electrodes will have an arcuate shape (when unconstrained) so that the electrode arrays deploy radially outwardly as the electrodes are advanced distally from the probe. In a particularly preferred configuration, the electrode arrays are xe2x80x9cevertingxe2x80x9d where the electrode tips first diverge radially outwardly and thereafter turn by more than 90xc2x0, often to 180xc2x0, or more, in the proximal direction. The deployed electrodes will usually define a generally cylindrical, conical, or spherical volume having a periphery with a maximum radius in the range from 0.5 to 3 cm.
The apparatus of the present invention maintains at least the distal tips of the individual electrodes within the cannula lumen with a substantially equal circumferential spacing therebetween with an annulus or annular envelope near the distal end of the cannula lumen. Usually, the spacing between adjacent electrodes at the distal tip of the cannula will vary by less than xc2x110%, preferably being less than xc2x15%, of the average spacing (i.e., total circumferential distance divided by the number of circumferentially deployed electrodes). Preferably, the electrodes will be maintained generally parallel to each other in the axial direction so that the initial entry of the electrodes into the tissue and subsequent passage of the electrodes through the tissue will be in substantially similar patterns (although in circumferentially spaced-apart directions). The annulus can be defined in a variety of ways, as described below.
In a first particular aspect of the present invention, the probe system comprises a core disposed coaxially within the cannula lumen to maintain substantially equal circumferential spacing between the electrodes when retracted in the cannula lumen. The core has a cylindrical outer surface which together with an inner surface of the cannula lumen defines the annulus or annular envelope for holding the electrodes. The annular envelope will preferably have a width in the radial direction which is less than or equal to twice the thickness of the electrodes, more preferably being less than or equal to 1.5 times the thickness. By limiting the width thusly, the electrodes will not be able to pass over each other and become misaligned while they are present in the annular envelope of the cannula. In the case of electrodes having unequal thickness, it will be necessary to limit the width to no more than the combined thicknesses of the two smallest adjacent electrodes to assure that adjacent electrodes cannot pass over each other. The electrodes, which are usually shaped to assume an arcuate configuration, are thus constrained within the annular envelope and held in a generally evenly spaced-apart manner. It has been found provision of such a core within the electrodes greatly increases the uniformity of penetration into tissue when compared to identical electrodes in the absence of a core in the cannula lumen.
The cannula core may be mechanically coupled to and reciprocate with the electrodes. In such case, it is desirable that the core have a sharpened distal end so that it can penetrate into tissue at the same time the electrodes penetrate into tissue. The core may be electrically coupled to the electrodes (in which case it acts as an additional electrode), or may be electrically isolated from the electrodes. It is possible that the core could act as a common or counter electrode in operating the probe in a bipolar manner. In a specific example, either the outer cylindrical surface of the core, the inner surface of the cannula, or both, may have axial channels formed therein for receiving and axially aligning the individual electrodes as the electrodes are advanced from the probe.
In a second particular aspect, the probe system comprises electrodes having asymmetric cross-sections with a major dimension aligned circumferentially and a minor dimension aligned radially. Usually, the major dimension will be at least 50% greater than the minor dimension, with such electrodes typically having a rectangular or trapezoidal cross-section. Electrodes having such cross-sectional dimensions are generally stiffer in the transverse direction and more flexible in the radial direction. By increasing transverse stiffness, proper circumferential alignment of the electrodes within an annulus within the open cannula lumen is enhanced. Exemplary electrodes will have a width (in the circumferential direction) in the range from 0.6 mm to 0.2 mm, preferably from 0.4 mm to 0.35 mm, and a thickness (in the radial direction) in the range from 0.05 mm to 0.3 mm, preferably from 0.1 mm to 0.2 mm. Often, the use of asymmetric electrodes will be sufficient in itself to provide for uniform electrode deployment, but in some instances it may be desirable to combine asymmetric electrode design with the use of a core and/or annular electrode envelope to maximize proper electrode alignment within the cannula.
In a third particular aspect of the apparatus of the present application, the electrodes may be closely packed or xe2x80x9cnestedxe2x80x9d within an annular electrode envelope within the lumen of the cannula. Usually, although not necessarily, such nested electrodes will have an asymmetric cross-section, as generally described above. By nesting the electrodes, uniform deployment of the electrode tips from the distal end of the cannula is assured. The use of nested electrodes may be combined with a cannula core, as described above. When the electrodes are nested, it will often be sufficient to extend the core only within the proximal portion of the cannula, leaving the distal core region empty and without structure. The absence of the core near the distal end of the cannula can be beneficial as it can facilitate electrode retraction back into the cannula, avoiding jamming caused by tissue trying to reenter a restricted annular envelope defined by the core within the cannula.
According to the method of the present invention, plurality of at least three electrodes is constrained within the cannula. The electrodes have distal tips arranged in a substantially equally spaced-apart pattern within an annular region near the distal end of the cannula. The annular region may be provided by any of the structures described above. The electrodes are advanced distally from the cannula into tissue at a target region within the tissue. By properly aligning the electrodes within the cannula, the electrodes deploy radially outwardly in a symmetric pattern into the tissue. The tissue is then treated by applying electrical current to the electrodes, typically radio frequency current, in a monopolar or bipolar fashion.
The annulus within the cannula may be defined between an outer cylindrical surface of a core member and an inner cylindrical surface of a cannula lumen. In such cases, the core member may be advanced into the tissue together with the electrodes or may be maintained stationary within the cannula as electrodes are advanced. Preferably, the electrodes have an asymmetric cross-section with a major dimension aligned circumferentially within the annular envelope and a minor dimension aligned radially within the annular envelope. Such electrodes are pre-shaped to bend about an axis parallel to the major dimension as the electrodes are advanced distally from the cannula into tissue, resulting in the preferred everting electrode array of the present invention.
Alternatively, the annulus within the cannula may be defined by nesting the electrodes in a closely packed pattern so that the desired alignment is maintained within the cannula lumen. Preferably, the electrodes are nested over at least 50% of their lengths, more preferably over 75% of their lengths, so that axial alignment of the adjacent electrodes is achieved.
In another aspect of the method of the present invention, electrodes are deployed in tissue by providing a plurality of at least three electrodes constrained within a cannula. Each of the electrodes is then advanced distally from the cannula into tissue, and spacing between adjacent electrodes is maintained to within xc2x120% of the average distance between electrodes at all times while the electrodes are advanced. Preferably, the spacing is maintained to within xc2x110% of the average distance, and more preferably to within xc2x15% of the average distance. Such uniform advancement of the electrodes may be achieved by any of the apparatus and methods described above.
In a still further aspect of the method of the present invention, electrodes are deployed into tissue by disposing at least two electrodes at a target site in or on the tissue. The electrodes are then advanced into the tissue while applying a RF current to the tissue through the electrodes. The RF current will be selected to have a voltage, power, and waveform which facilitate passage of the electrodes through the tissue by reducing resistance to electrode advancement. The application of a xe2x80x9cdeploymentxe2x80x9d current to the electrodes can be used as an alternative to or in addition to sharpening of the distal tips of the electrodes to facilitate passage through tissue. After the electrodes are deployed in a uniform manner, as described above, an xe2x80x9cablationxe2x80x9d current can be applied through the electrodes to treat the tissue. Suitable deployment currents will have a voltage in the range from 50V to 200V (peak-to-peak), usually from 50V to 100V, and a power in the range from 100 W to 300 W, usually from 100 W to 200 W. The waveform is not critical. Suitable ablation currents will have a voltage below 150V (peak-to-peak), usually being from 50V to 100V. The power will usually be from 40 W to 100 W with a sine wave form.